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Vacuum Heat Treatments for Steel

Low-alloy high-speed steel is a new type of steel that appeared in the late 1970s internationally. Due to the use of the latest contemporary alloying principles to determine a reasonable chemical composition, it has good performance and low quenching temperature. Moreover, because of the reduction of the amount of alloying elements in the steel, the cost is reduced, and the price is 15% to 20% lower than that of ordinary high-speed steel, which is very attractive to tool manufacturers. However, there is a lack of in-depth research on the basic characteristics and application effects of F205 steel, so this steel has not been accepted by most tool manufacturers. This paper studies and analyzes the optimal heat treatment process of F205 steel spiral flute taps, and lays the foundation for mass production of special-shaped cutting tools by vacuum heat treatment process.

1 Vacuum pressurized gas quenching process

The heating temperatures of the selected F205 steel were 1140C, 1150C, 1160C and 1170C, respectively. Preheat at 850°C during the heating process to reduce the temperature difference, reduce the deformation of the tap, and shorten the high temperature residence time. Quenching and cooling is pure nitrogen with a pressure of 4.7×10⁵Pa.

2 vacuum tempering process

In order to understand the changes in microstructure and properties of F205 steel after vacuum quenching and tempering, the tempering temperatures were selected to be 100C, 200C, 300C, 400C, 500C, 560C, 600C, 700Cx1h, and three times of vacuum tempering. Test the hardness after vacuum tempering to determine the best process parameters.

3 red hardness test

The hardness of the F205 steel sample was vacuum quenched at 560 C x 3 times at different temperatures, and then heated at 600 C x 1h x 3 times to determine its hardness. Metallographic analysis with a microscope shows that the grain size of the quenched austenite in F205 steel (that is, the grain boundary).

4 Vacuum heat treatment results and analysis

The annealed structure of the raw material is fine carbides distributed on the spherical sorbite matrix, and its longitudinal direction is slightly banded, but not serious, which meets the technical requirements of the manufacturer. The alloying elements contained in F205 steel mainly exist in the form of carbides. According to X-ray diffraction, its structure is complex and there are many types, mainly (FeW)₃C and (FeMo)₃C, followed by (FeW)₆C, (FeMo)₆C and WC, MoC, Cr₂₃C₆, Cr₂C₃, V₄C₃ and so on. These carbides can only be fully dissolved into the austenite at sufficiently high temperatures. This is what should be paid attention to when determining the vacuum quenching process.

4.1 Microstructure and properties of vacuum quenching

As the vacuum quenching heating temperature increases from 1140°C to 1170C, the austenite grains gradually become thicker, and the undissolved carbides decrease, and their pinning effect on the grain boundary migration and expansion when the grains grow is weakened. The crystal grains grow abnormally and appear mixed crystal phenomenon.

As the vacuum quenching temperature increases, the hardness of the sample decreases continuously, such as 64.7HRC at 1140C~1150C and 62.9HRC at 1170C. This is because the increase in heating temperature increases the solid solubility of austenite alloy elements, resulting in Retained austenite and grain growth appear after quenching.

According to the fracture morphology of F205 steel quenched. Vacuum quenching at 1150C~1160C exhibits a fine porcelain-like quasi-cleavage fracture, which is characterized by small quasi-cleavage platforms and tear edges. There are obvious cleavage fractures in the part of the fracture surface quenched at 1170C, which may correspond to the mixed crystal phenomenon, which increases the brittleness of the steel and reduces the strength and toughness. At higher multiples, the grain boundaries after quenching at any temperature are relatively pure and clear. No precipitation of salt-and-pepper carbides. This shows that the cooling capacity of pressurized gas quenching is sufficient.

4.2 Vacuum tempering structure and properties

Similar to general high-speed steel, the hardness of F205 steel changes with the increase of tempering temperature after vacuum quenching at different temperatures. It can be seen that the peak value of tempering hardness at 560C is (65-66) HRC, which is not inferior to M₂ steel. After tempering at 560C x1h×3 times, the quenched martensite transforms into tempered martensite, and a large number of dispersed alloy carbides precipitate; at the same time, most of the retained austenite transforms into tempered martensite, so secondary hardening occurs Phenomenon. The relaxation and elimination of quenching stress can significantly improve the strength and toughness of steel.